The Segregation Behavior of Phosphorus in Ti and Ti+Nb Stabilized Interstitial-Free Steels.

Abstract

The phenomenon of cold work embrittlement (CWE) in the P-added high strength interstitial-free steels has been associated with the segregation of P to the ferrite grain boundaries. This segregation by P is believed to decrease the cohesive strength of the grain boundaries by weakening their bonding. Hence, the resistance of the steel to brittle fracture, i.e., intergranular fracture, is greatly decreased. The goal of the present study was to investigate the segregation behavior of P during the different stages of processing (prior to and after coiling, and after cold rolling and annealing) in Ti and Ti+Nb stabilized interstitial-free steels. It was found that a considerable amount of segregation of P to the ferrite grain boundaries occurred during the coiling process in Ti-stabilized interstitial-free steels. However, with the addition of Nb in the Ti+Nb-stabilized interstitial-free steels, the segregation of P was decreased in the as-coiled condition. The P content on the ferrite grain boundaries in the final cold rolled and annealed condition was found to depend on two factors; (1) the segregation of P in the as-coiled condition, and (2) availability of Ti to form phosphides during the annealing process. It was confirmed in this study that the addition of P decreases the CWE resistance of the steel. Furthermore, it appears that the CWE resistance of the Ti+Nb-stabilized interstitial-free steels is improved by the presence of solute Nb on the ferrite grain boundaries.